In recent years, switching power supplies are developing to be miniaturized. Following such a development, switching power supplies are designed with higher switching frequency, and thus the volume of magnetic elements in the switching power supplies can be reduced. For Alternating Current (AC) input application, there is usually a Bulk capacitor to balance the instantaneous input and output powers under line frequency, and thus the Bulk capacitor cannot be miniaturized as the switching frequency increases. As a result, as the switching frequency increases, volume of the Bulk capacitor occupies a growing proportion in the volume of a switching power supply, which becomes a bottleneck of the miniaturization of switching power supplies.